1. Field of the Invention
The present invention relates to a lens, and in particular relates to a zoom lens having increased positioning accuracy.
2. Description of the Related Art
Referring to FIGS. 1 to 3, Taiwan Application No. 95116393 discloses a lens configuration which includes a lens 8 retractable along an axial line L and a control system 90 controlling the lens 8.
The lens 8 includes a base 81, a guide roll 82 rotatably jacketed inside the base 81, an inner roll 83 disposed in and rotated relative to the guide roll 82, a lens chamber unit 84 disposed in the guide roll 82 and rotated/moved relative to the inner roll 83, and a driving unit 85 utilized to perform extension/retraction movements of the lens 8. The driving unit 85 includes a motor 851 disposed on the base 81 and a decelerating gear system 852 operatively connected to the motor 851 and the guide roll 82.
The control system 90 includes a position detection unit 91, a zero detection unit 92, a micro-processing unit 93 connected to the position detection unit 91 and the zero detection unit 92, and a motor control unit 94 connected to the micro-processing unit 93 and the motor 851. The position detection unit 91 includes an impeller 911 disposed on the base 81 and rotationally driven by the motor 851 and a first induction element 912 disposed on the base 81 capable of sensing the rotation of the impeller 911. The zero detection unit 92 includes a reflective sheet 921 attached to the outside of the guide roll 82 and a second induction element 922 disposed on the base 81 capable of sensing the reflective sheet 921.
When the lens 8 controlled by the control system 90 is extended or retracted to perform the zooming process to cause the second induction element 922 to sense the reflective sheet 921, a zero point signal outputted from the second induction element 922 is provided for the micro-processing unit 93 to determine the position of the zero point and to reset the rotation count of the impeller 911. When the motor 851 keeps rotating, the rotation of the impeller 911 can be sensed by the first induction element 912, and a rotation signal can be provided for the micro-processing unit 93 to determine the rotation count of the impeller 911 for obtaining an extended position of the lens 8.
During the zooming process of the lens 8, the extended position of the lens 8 can be calculated by the rotation count of the impeller 911 after the zero point is determined by the micro-processing unit 93. Due to the improper motion of the impeller 911 during the zooming process, the rotation count sensed by the first induction element 912 cannot be accurately calculated, and the actual extension/retraction position of the lens 8 cannot be accurately controlled by the micro-processing unit 93. In particular, when the variation of the focus range of the lens 8 is large or the frequency of the reciprocal extension/retraction of the lens 8 during the zooming process is high, the accumulated error due to the rotation count of the impeller 911 will affect the positioning accuracy of the lens 8.